1. Field of the Invention
The present invention relates to vehicular antennas and more particularly to retractable antennas adapted to receive AM/FM radio signals and to receive and transmit higher-frequency signals, such as cellular telephone signals.
2. Description of the Prior Art
Cellular telephone service is becoming exceedingly popular and is very much in demand. Since cellular telephones operate in a frequency band considerably higher than the normal AM/FM radio, separate cellular telephone antennas must be installed on vehicles. Initially the existence of the cellular antenna on a vehicle was a status symbol but it is now considered a pretentious display that is to be avoided by those in the service industry. Automobile owners dislike the unsightly objects extending from their vehicles and the need for multiple feed cable holes in the vehicle's exterior for body mounted antennas In addition, cellular telephones are common targets for thieves, and the cellular antenna is literally a flag directing potential thieves to the desired vehicles.
It is desirable to retract a radio antenna into the body of the vehicle so as to leave the vehicle's lines clean and streamline when the radio is not in use. Retractable antennas are also desirable since the antennas, if they are not retractable, are commonly damaged when the vehicle passes through a car wash. Electrically powered mechanisms for retracting AM/FM radio antennas have become quite common on most modern vehicles. The same feature would be extremely desirable for a cellular telephone antenna.
It is also desirable to provide a single multiband antenna which can handle both the AM/FM commercial broadcast frequencies and the cellular telephone frequencies. Multiband antennas have been provided for use with citizen band (CB) radios as illustrated in U.S. Pat. Nos. 4,095,229 and 4,325,069. Such antennas may be coupled through a single feed line to a splitter to separate the AM/FM and citizen band (CB) radio frequencies. In other situations, a loading coil is provided on the antenna itself to produce an effective length suitable for transmission and reception of the desired frequency band.
Retractable triband antennas for the AM/FM bands and the cellular telephone band are disclosed in U.S. Pat. Nos. 4,647,941; 4,658,260; 4,675,687; 4,721,965; 4,748,450 and 4,847,629.
The numerous devices of the prior art provide triband antennas for AM/FM reception and cellular telephone service; however, in general the prior art antennas exhibit a high voltage standing wave ratio (VSWR), poor isolation between the cellular and AM/FM antenna portions, a radiation pattern off the horizontal axis, poor impedance and pattern bandwidth.